Chromium steel sheets having an excellent press formability

ABSTRACT

A chromium steel sheet having excellent press formability, particularly deep-drawing formability and resistance to secondary work brittleness. The construction is a chromium steel sheet including C: not more than 0.03 wt %, Si: not more than 1.0 wt %, Mn: not more than 1.0 wt %, P: not more than 0.05 wt %, S: not more than 0.015 wt %, Al: not more than 0.10 wt %, N: not more than 0.02 wt %, Cr: 5-60 wt %, Ti: 4(C+N)-0.5 wt %, Nb: 0.003-0.020 wt %, B: 0.0002-0.005 wt %, and, if necessary, one or more selected from Mo: 0.01-5.0 wt %, Ca: 0.0005-0.01 wt % and Se: 0.0005-0.025 wt %, and the balance being Fe and inevitable impurities.

TECHNICAL FIELD

This invention relates to chromium steel sheets (inclusive of steelstrips) having an excellent press formability, particularly excellentdeep-drawing formability and resistance to secondary workingbrittleness.

BACKGROUND ART

As a typical kind of the chromium steel sheets, ferritic stainless steelsheets are usually produced through steps of hot rolling--annealing ofhot rolled sheet--cold rolling--finish annealing after the heating ofcontinuously cast slab.

In general, the thus produced ferritic stainless steel is excellent inthe resistance to stress corrosion cracking and is cheap, so that it iswidely used to applications such as various kitchenwares, automobileparts and the like. However, the steel is often subjected to a severerdeep drawing in the application such as fuel filter casing forautomobile and the like, so that there is frequently caused a problem ofcreating cracks due to secondary working brittleness.

Therefore, there have made many attempts in order to improve thedeep-drawing formability and the resistance to secondary workingbrittleness in the ferritic stainless steel sheets.

For example, JP-B-54-11770 has proposed a production technique offerritic stainless steel sheets aiming at a high cold workability byaddition of Ti, while JP-B-57-55787 has proposed a production techniqueof ferritic stainless steel sheets aiming at a high Lankford value(hereinafter abbreviated as "r-value") by addition of B. Furthermore,JP-B-2-7391 has proposed a production technique for ferritic stainlesssteel sheets which limits brittle cracks after deep drawing by additionof Ti and B.

However, these techniques have problems as mentioned below. That is, thebrittle cracks are frequently observed at the secondary working afterseverer deep drawing in the technique disclosed in JP-B-54-11770.Further, the technique disclosed in JP-B-57-55787 is unsuitable forseverer deep drawing because the deep drawability is insufficient. Andalso, the addition of Ti and B is conducted in the technique disclosedin JP-B-2-7391, but either deep drawability or resistance to secondarywork brittleness is poor and both the properties are not simultaneouslysatisfied. Moreover, these techniques have a problem in that the planeanisotropy of r-value (hereinafter abbreviated as "Δr") is notsufficiently improved.

As mentioned above, all of the above techniques improve either thedeep-drawing formability or the resistance to secondary workembrittlement, but have the common problem that both the properties arenot simultaneously improved. Therefore, the occurrence of brittle cracksin the subsequent secondary working is still a concern after severerdeep drawing.

It is, therefore, an object of the invention to provide chromium steelsheets having excellent press formability, particularly deep-drawingformability and resistance to secondary work brittleness.

It is another object of the invention to provide chromium steel sheetshaving an r-value of not less than 1.5, Δr of not more than 0.3 and abrittle crack creating temperature of not higher than -50° C.

DISCLOSE OF INVENTION

The inventors have made various studies in order to achieve the aboveobjects and found that the deep-drawing formability and the resistanceto secondary work brittleness are simultaneously improved and furtherthe ductility of weld portion is improved by controlling the chemicalcomposition of the chromium steel sheet to a proper range, and as aresult the invention has been accomplished.

The chromium steel sheet having the above properties has the followingconstruction:

(1) The invention is a chromium steel sheet comprising;

C: not more than 0.03 wt %, Si: not more than 1.0 wt %,

Mn: not more than 1.0 wt %, P: not more than 0.05 wt %,

S: not more than 0.015 wt %, Al: not more than 0.10 wt %,

N: not more than 0.02 wt %, Cr: 5-60 wt %,

Ti: 4(C+N)-0.5 wt %, Nb: 0.003-0.020 wt %,

B: 0.0002-0.005 wt %,

and the balance being Fe and inevitable impurities.

(2) The invention is a chromium steel sheet further containing Mo:0.01-5.0 wt % in addition to the main ingredient of the above item (1).

(3) The invention is a chromium steel sheet further containing Ca:0.0005-0.01 wt % in addition to the main ingredient of the above item(1).

(4) The invention is a chromium steel sheet further containing Se:0.0005-0.025 wt % in addition to the main ingredients of the above item(1).

(5) The invention is a chromium steel sheet further containing Mo:0.01-5.0 wt % and Ca: 0.0005-0.01 wt % in addition to the mainingredient of the above item (1).

(6) The invention is a chromium steel sheet further containing Mo:0.01-5.0 wt % and Se: 0.0005-0.025 wt % in addition to the mainingredient of the above item (1).

(7) The invention is a chromium steel sheet further containing Ca:0.0005-0.01 wt % and Se: 0.0005-0.025 wt % in addition to the mainingredients of the above item (1).

(8) The invention is a chromium steel sheet further containing Mo:0.01-5.0 wt %, Ca: 0.0005-0.01 wt % and Se: 0.0005-0.025 wt % inaddition to the main ingredients of the above item (1).

(9) The invention is a chromium steel sheet wherein Mo content in anyoneof the above items (2), (5), (6) and (8) is 0.1-3.0 wt %.

(10) The invention is a chromium steel sheet wherein a relationshipbetween Ti content and Nb content in anyone of the above items (1)-(9)satisfies Ti/Nb≧7.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing the influence of Nb content upon Δr;

FIG. 2 is a graph showing the relationship between r-value and crackcreating temperature; and

FIG. 3 is a diagrammatical view illustrating a method of repetitivebending test.

BEST MODE FOR CARRYING OUT THE INVENTION

The preferable conditions for carrying out the invention will bedescribed below.

The chromium steel sheets according to the invention explained in theabove item "DISCLOSURE OF INVENTION" are excellent in the pressformability, particularly the deep-drawing formability and resistance tosecondary work brittleness, and satisfy the r-value of not less than1.5, the Δr of not more than 0.3 and the brittle crack creatingtemperature of not higher than -50° C.

The action of each ingredient element and the reason on the numericallimitation in the invention will be described below.

    C: not more than 0.03 wt %;

C is an element lowering the r-value and elongation property.Particularly, when it exceeds 0.03 wt %, the influence is conspicuous,so that the content is necessary to be not more than 0.03 wt %.Preferably, it is not more than 0.01 wt %.

    Si: not more than 1.0 wt %;

Si is an element effective for deoxidation. The excessive additionbrings about the degradation of the cold workability, so that theaddition range is not more than 1.0 wt %, preferably not more than 0.5wt %.

    Mn: not more than 1.0 wt %;

Mn is an element effective for precipitating and fixing S existent inthe steel to maintain the hot rolling property. The excessive additionbrings about the degradation of the cold workability, so that theaddition range is not more than 1.0 wt %, preferably not more than 0.5wt %.

    P: not more than 0.05 wt %;

P is an element harmful for hot workability. Particularly, when itexceeds 0.05 wt %, the influence becomes conspicuous, so that thecontent is not more than 0.05 wt %, preferably not more than 0.04 wt %.

    S: not more than 0.015 wt %;

S segregates in a crystal grain boundary to promote grain boundarybrittleness and is a harmful element. Particularly, when it exceeds0.015 wt %, the influence becomes conspicuous, so that the content isnot more than 0.015 wt %, preferably not more than 0.008 wt %.

    Al: not more than 0.10 wt %;

Al is an element effective for deoxidation. The excessive additionbrings about the surface defect due to the increase of Al inclusions, sothat the content is not more than 0.10 wt %, preferably not more than0.07 wt %.

    N: not more than 0.02 wt %;

N is an element harmful for the deep-drawing formability likewise C.Particularly, when it exceeds 0.02 wt %, the influence becomesconspicuous, so that the content is necessary to be not more than 0.02wt %. Preferably, it is not more than 0.01 wt %.

    Cr: 5-60 wt %;

Cr is an element necessary for ensuring the corrosion resistance as thestainless steel. When the content is less than 5 wt %, the corrosionresistance is lacking, while when it exceeds 60 wt %, the coldworkability is degraded, so that the addition range is 5-60 wt %,preferably 10-45 wt %.

    Ti: 4(C+N)-0.5 wt %;

Ti is an element useful for precipitating and fixing C, N harmful fordeep-drawing formability to ensure highly deep-drawing formability. Theeffect is not obtained in an amount of less than 4(C+N) wt %, while theeffect is saturated and the productivity lowers when it exceeds 0.5 wt%. Therefore, the addition amount of Ti is 4(C+N)-0.5 wt %, preferably4(C+N)-0.3 wt %.

    Nb: 0.003-0.020 wt %;

Nb is an element particularly important for simultaneously improving thedeep-drawing formability and the resistance to secondary workbrittleness by composite addition with Ti, B and the like in theinvention. The effect is not obtained in an amount of less than 0.003 wt%, while the effect is saturated and the production cost is ratherincreased when it exceeds 0.020 wt %. The addition amount of Nb is0.003-0.020 wt %, preferably 0.004-0.018 wt %.

The effect of Nb on the deep-drawing formability and the resistance tosecondary work brittleness is explained in detail with reference to thefigures. FIG. 1 shows the influence of Nb on Δr in a cold rolled steelsheet (cold reduction through work rolls having a roll diameter of notless than 150 mm: 82.5%) containing (0.007-0.009)wt % C-(0.3-0.4)wt %Si-(0.3-0.4)wt % Mn-(0.02-0.03)wt % P-(0.005-0.007)wt % S-(0.02-0.03)wt% Al-(0.0070-0.0090)wt % N-(16-18)wt % Cr-(0.15-0.17)wt %Ti-(0.0008-0.0010)wt % B. From FIG. 1, it is apparent that Δr isconsiderably improved by adding Nb of not less than 0.003 wt % and hencethe edge shape after the deep drawing is largely improved.

Further, FIG. 2 shows the influence of Nb amount upon the relationshipbetween brittle cracking and r-value after secondary working of a coldrolled steel sheet (cold reduction through work rolls having a rolldiameter of not less than 150 mm: 82.5%) containing (0.007-0.009)wt %C-(0.3-0.4)wt % Si-(0.3-0.4)wt % Mn-(0.02-0.03) wt % P-(0.005-0. 007) wt% S-(0.02-0.03) wt % Al-(0.0070-0.0090)wt % N-(16-18)wt %Cr-(0.15-0.17)wt % Ti-(0.001-0.018)wt % Nb-(0.0008-0.0010)wt % B. FromFIG. 2, it is apparent that the steel sheets containing not less than0.003 wt % of Nb are high in the r-value as a forming limit indicationin the deep drawing and low in the brittle crack creating temperature.

As mentioned above, both the deep-drawing formability and the resistanceto secondary work brittleness are shown to be balanced at a high levelby including not less than 0.003 wt % of Nb.

    Ti/Nb≧7

The press formability is improved by composite addition of Ti and Nbinstead of single addition. Particularly, Δr is considerably small whenTi and Nb are added together, which acts to considerably improve thepress formability. This effect can more surely be attained by thecomposite addition of Ti and Nb under a condition satisfying Ti/Nb≧7.

    B: 0.0002-0.005 wt %;

B is an element effective for improving the resistance to secondary workbrittleness after the deep drawing. The effect is not obtained in anamount of less than 0.0002 wt %, while the excessive addition degradesthe deep-drawing formability. The addition amount is 0.0002-0.005 wt %,preferably 0.0003-0.003 wt %.

    Mo: 0.01-5.0 wt %, preferably 0.1-3.0 wt %;

Mo is an element improving the press formability (r-value, Δr,resistance to secondary work brittleness) and the corrosion resistance,and is added selectively. The improvement of r-value and Δr by theaddition of Mo is due to the fact that the recrystallization grainelongation rate is near to 1 together with the fine formation ofrecrystallization grains in the annealed sheet. The effect,is obtainedin an amount of not less than 0.01 wt %, but the addition exceeding 5.0wt % brings about the degradation of deep-drawing formability, so thatthe addition amount of Mo is 0.01-5.0 wt %. Moreover, the preferableaddition amount is 0.1-3.0 wt %.

    Ca: 0.0005-0.01 wt %

Ca is an element having an effect of controlling nozzle clogging with Tiinclusion in the steel making and casting and is selectively added inaccordance with the Ti content. However, when Ca is excessively added,Ca inclusion is a starting point of brittle breakage, so that theaddition range of Ca is 0.0005-0.01 wt %, preferably 0.0005-0.006 wt %.

    Se: 0.0005-0.025 wt %

Se is an important element enhancing the flowability of welded metal inthe welding to control surface defects (cracking) of weld portions andimprove the ductility of the weld portions. This effect appears in anamount of not less than 0.0005 wt %, but when it exceeds 0.025 wt %, thecorrosion resistance lowers, so that the addition range of Se is0.0005-0.025 wt %, preferably 0.0008-0.010 wt %.

The object of the invention is attained by the above chemicalingredients, but the effect of the invention is not lost even if0.01-0.5 wt % of V, 0.3-6 wt % of Ni, 0.3-6 wt % of Co, 0.1-3 wt % ofCu, 0.3-6 wt % of W are added in addition to these ingredients.

The production of the steel sheet according to the invention may becarried out by a method wherein steel having the above chemicalcomposition is melted in a usual steel-making furnace such as aconvertor, electric furnace or the like, shaped into a steel slab bycontinuous casting process or steel ingot process, and then subjected tohot rolling--(annealing of hot rolled sheet)--pickling--coldrolling--annealing of cold rolled sheet--pickling--if necessary,repetition of cold rolling--annealing--pickling.

However, the object can more advantageously be attained when the rollsdiameter of cold rolling work roll and the reduction of cold rolling arecontrolled to roll diameters of: not less than 150 mm, preferably250-1000 mm, and reduction: not less than 30%, preferably 40-95% amongcold rolling conditions in the above cold rolling step. That is, thecold rolled stainless steel sheet is generally rolled through work rollshaving a roll diameter of not more than 100 mm. When the roll diameteris made larger as mentioned above, the shearing stress in the rollingdirection through friction between the roll and the steel sheet surfaceis mitigated and also the difference of stress in the sheet surfacebecomes small. As a result, the r-value and Δr can be more improvedwithout degrading the resistance to secondary work brittleness. In thiscase, when the roll diameter is less than 150 mm, or when the reductionis less than 30% even if the roll diameter is not less than 150 mm, theeffect is insufficient, while when the roll diameter exceeds 1000 mm,the power required for driving such a roll becomes excessive andeconomically disadvantageous, and if the reduction through this rollexceeds 95%, the surface properties tend to be degraded due to thesticking between the roll and the steel sheet.

EMBODIMENTS EXAMPLE 1

A steel having a chemical composition as shown in Tables 1, 2, and 3 ismelted in a convertor and rendered into a steel slab through secondaryrefining, which was heated to 1250° C. and hot rolled to obtain a hotrolled sheet having a thickness of 4.0 mm. The hot rolled sheet wassubjected to annealing of hot rolled sheet (800°-950°)--pickling--coldrolling--annealing of cold rolled sheet (800°-950° C.)--pickling toobtain a cold rolled steel sheet having a thickness of 0.7 mm.

The deep-drawing formability (r-value, Δr) and the resistance tosecondary work brittleness were measured with respect to the steelsheets obtained by the above method as a test specimen, and theductility of weld portion was measured with respect to a part of thesteel sheets according to the following method.

                                      TABLE 1                                     __________________________________________________________________________    Chemical composition (wt %)                                                   Steel                                                                            C  Si Mn P  S  AL N   Cr Ti Nb B   Ca  Mo 4(C + N)                                                                           Ti/Nb                                                                            Remarks                  __________________________________________________________________________    1  0.011                                                                            0.43                                                                             0.46                                                                             0.032                                                                            0.004                                                                            0.030                                                                            0.0115                                                                            11.2                                                                             0.153                                                                            0.005                                                                            0.0006                                                                            0.0015                                                                            -- 0.0900                                                                             31 Accept-                  2  0.010                                                                            0.21                                                                             0.32                                                                             0.037                                                                            0.006                                                                            0.025                                                                            0.0091                                                                            10.9                                                                             0.206                                                                            0.012                                                                            0.0008                                                                            --  -- 0.0764                                                                             17 able                     3  0.010                                                                            0.39                                                                             0.28                                                                             0.021                                                                            0.008                                                                            0.031                                                                            0.0081                                                                            16.7                                                                             0.103                                                                            0.008                                                                            0.0004                                                                            0.0023                                                                            -- 0.0724                                                                             13 Examples                 4  0.014                                                                            0.62                                                                             0.19                                                                             0.018                                                                            0.005                                                                            0.046                                                                            0.0088                                                                            16.9                                                                             0.151                                                                            0.017                                                                            0.0005                                                                            0.0018                                                                            -- 0.0912                                                                             9                           5  0.014                                                                            0.20                                                                             0.26                                                                             0.017                                                                            0.011                                                                            0.002                                                                            0.0072                                                                            17.3                                                                             0.161                                                                            0.003                                                                            0.0011                                                                            0.0006                                                                            0.13                                                                             0.0848                                                                             54                          6  0.009                                                                            0.18                                                                             0.30                                                                             0.019                                                                            0.005                                                                            0.025                                                                            0.0066                                                                            17.0                                                                             0.149                                                                            0.010                                                                            0.0008                                                                            --  0.98                                                                             0.0624                                                                             15                          7  0.010                                                                            0.31                                                                             0.15                                                                             0.029                                                                            0.004                                                                            0.001                                                                            0.0076                                                                            16.8                                                                             0.152                                                                            0.018                                                                            0.0015                                                                            0.0030                                                                            -- 0.0704                                                                             8                           8  0.007                                                                            0.15                                                                             0.56                                                                             0.029                                                                            0.013                                                                            0.031                                                                            0.0085                                                                            17.2                                                                             0.093                                                                            0.004                                                                            0.0025                                                                            0.0041                                                                            0.31                                                                             0.0620                                                                             23                          9  0.012                                                                            0.20                                                                             0.23                                                                             0.030                                                                            0.004                                                                            0.029                                                                            0.0034                                                                            17.0                                                                             0.131                                                                            0.009                                                                            0.0040                                                                            0.0020                                                                            -- 0.0616                                                                             15                          10 0.010                                                                            0.20                                                                             0.17                                                                             0.024                                                                            0.007                                                                            0.038                                                                            0.0088                                                                            16.5                                                                             0.283                                                                            0.006                                                                            0.0010                                                                            0.0053                                                                            -- 0.0752                                                                             37                          11 0.015                                                                            0.19                                                                             0.42                                                                             0.018                                                                            0.006                                                                            0.056                                                                            0.0089                                                                            17.2                                                                             0.309                                                                            0.013                                                                            0.0008                                                                            0.0080                                                                            -- 0.0956                                                                             24                          12 0.006                                                                            0.23                                                                             0.15                                                                             0.023                                                                            0.002                                                                            0.023                                                                            0.0063                                                                            30.5                                                                             0.103                                                                            0.007                                                                            0.0004                                                                            --  -- 0.0492                                                                             15                          13 0.002                                                                            0.32                                                                             0.19                                                                             0.042                                                                            0.002                                                                            0.007                                                                            0.0071                                                                            39.2                                                                             0.162                                                                            0.015                                                                            0.0015                                                                            0.0015                                                                            -- 0.0364                                                                             11                          14 0.007                                                                            0.17                                                                             0.13                                                                             0.020                                                                            0.002                                                                            0.013                                                                            0.0090                                                                            51.3                                                                             0.125                                                                            0.010                                                                            0.0007                                                                            0.0021                                                                            -- 0.0640                                                                             13                          15 0.011                                                                            0.26                                                                             0.21                                                                             0.019                                                                            0.004                                                                            0.011                                                                            0.0073                                                                            17.0                                                                             0.231                                                                            0.005                                                                            0.0006                                                                            0.0010                                                                            2.56                                                                             0.0732                                                                             46                          16 0.005                                                                            0.31                                                                             0.25                                                                             0.020                                                                            0.005                                                                            0.009                                                                            0.0081                                                                            30.0                                                                             0.119                                                                            0.010                                                                            0.0009                                                                            0.0019                                                                            0.79                                                                             0.0524                                                                             12                          __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________    Chemical composition (wt %)                                                   Steel                                                                            C  Si Mn P  S  AL N   Cr Ti Nb B   Ca  Mo Se  4(C + N)                                                                           Ti/Nb                                                                             Remarks             __________________________________________________________________________    17 0.007                                                                            0.30                                                                             0.30                                                                             0.021                                                                            0.005                                                                            0.020                                                                            0.0076                                                                            7.0                                                                              0.153                                                                            0.011                                                                            0.0009                                                                            0.0010                                                                            -- --  0.0608                                                                             14  Accept-             18 0.005                                                                            0.23                                                                             0.23                                                                             0.018                                                                            0.005                                                                            0.006                                                                            0.0103                                                                            11.3                                                                             0.153                                                                            0.009                                                                            0.0011                                                                            --  0.02                                                                             --  0.0612                                                                             17  able                19 0.010                                                                            0.10                                                                             0.18                                                                             0.023                                                                            0.009                                                                            0.030                                                                            0.0073                                                                            11.1                                                                             0.224                                                                            0.015                                                                            0.0009                                                                            --  0.21                                                                             --  0.0692                                                                             15  Examples            20 0.008                                                                            0.35                                                                             0.40                                                                             0.011                                                                            0.006                                                                            0.028                                                                            0.0059                                                                            18.0                                                                             0.126                                                                            0.010                                                                            0.0020                                                                            --  0.03                                                                             --  0.0556                                                                             13                      21 0.020                                                                            0.26                                                                             0.26                                                                             0.030                                                                            0.008                                                                            0.035                                                                            0.0121                                                                            17.9                                                                             0.253                                                                            0.015                                                                            0.0015                                                                            --  2.11                                                                             --  0.1284                                                                             17                      22 0.009                                                                            0.22                                                                             0.30                                                                             0.019                                                                            0.007                                                                            0.022                                                                            0.0054                                                                            30.2                                                                             0.103                                                                            0.007                                                                            0.0013                                                                            --  0.04                                                                             --  0.0576                                                                             15                      23 0.005                                                                            0.25                                                                             0.15                                                                             0.026                                                                            0.006                                                                            0.015                                                                            0.0103                                                                            30.3                                                                             0.159                                                                            0.015                                                                            0.0006                                                                            --  0.53                                                                             --  0.0612                                                                             11                      24 0.016                                                                            0.19                                                                             0.41                                                                             0.026                                                                            0.007                                                                            0.030                                                                            0.0083                                                                            18.0                                                                             0.162                                                                            0.011                                                                            0.0013                                                                            --  0.05                                                                             0.0013                                                                            0.0972                                                                             15                      25 0.015                                                                            0.23                                                                             0.26                                                                             0.030                                                                            0.005                                                                            0.016                                                                            0.0099                                                                            17.9                                                                             0.190                                                                            0.009                                                                            0.0009                                                                            --  2.03                                                                             0.0025                                                                            0.0996                                                                             21                      26 0.009                                                                            0.19                                                                             0.22                                                                             0.028                                                                            0.005                                                                            0.017                                                                            0.0039                                                                            17.6                                                                             0.151                                                                            0.006                                                                            0.0018                                                                            0.0015                                                                            -- 0.0023                                                                            0.0516                                                                             25                      27 0.005                                                                            0.26                                                                             0.31                                                                             0.015                                                                            0.004                                                                            0.011                                                                            0.0041                                                                            18.2                                                                             0.126                                                                            0.009                                                                            0.0020                                                                            0.0020                                                                            0.03                                                                             0.0011                                                                            0.0364                                                                             14                      28 0.009                                                                            0.10                                                                             0.11                                                                             0.030                                                                            0.006                                                                            0.009                                                                            0.0093                                                                            18.0                                                                             0.181                                                                            0.015                                                                            0.0007                                                                            0.0010                                                                            1.84                                                                             0.0052                                                                            0.0732                                                                             12                      29 0.011                                                                            0.36                                                                             0.38                                                                             0.022                                                                            0.006                                                                            0.023                                                                            0.0086                                                                            16.9                                                                             0.201                                                                            0.007                                                                            0.0011                                                                            --  -- 0.0008                                                                            0.0784                                                                             29                      30 0.009                                                                            0.33                                                                             0.29                                                                             0.028                                                                            0.005                                                                            0.022                                                                            0.0043                                                                            17.0                                                                             0.180                                                                            0.006                                                                            0.0008                                                                            0.0009                                                                            2.12                                                                             0.0029                                                                            0.0532                                                                             30                      31 0.011                                                                            0.36                                                                             0.41                                                                             0.022                                                                            0.006                                                                            0.023                                                                            0.0080                                                                            11.0                                                                             0.153                                                                            0.009                                                                            0.0013                                                                            --  -- 0.0013                                                                            0.0760                                                                             17                      __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________    Chemical composition (wt %)                                                   Steel                                                                            C  Si Mn P  S  AL N   Cr Ti Nb B   Ca  Mo 4(C + N)                                                                           Ti/Nb                                                                            Remarks                  __________________________________________________________________________    32 0.017                                                                            0.25                                                                             0.42                                                                             0.025                                                                            0.007                                                                            0.051                                                                            0.0086                                                                            11.0                                                                             0.142                                                                            0.001                                                                            0.0007                                                                            0.0023                                                                            -- 0.1024                                                                             142                                                                              Compara-                 33 0.013                                                                            0.41                                                                             0.19                                                                             0.028                                                                            0.004                                                                            0.026                                                                            0.0060                                                                            16.8                                                                             0.128                                                                            0.001                                                                            0.0001                                                                            0.0023                                                                            -- 0.0760                                                                             128                                                                              tive                     34 0.015                                                                            0.23                                                                             0.25                                                                             0.019                                                                            0.004                                                                            0.030                                                                            0.0073                                                                            17.0                                                                             0.133                                                                            0.001                                                                            0.0003                                                                            0.0018                                                                            -- 0.0892                                                                             133                                                                              Examples                 35 0.011                                                                            0.36                                                                             0.31                                                                             0.023                                                                            0 005                                                                            0.005                                                                            0.0088                                                                            17.1                                                                             0.129                                                                            0.002                                                                            0.0010                                                                            --  0.13                                                                             0.0792                                                                             129                         36 0.009                                                                            0.25                                                                             0.26                                                                             0.022                                                                            0.008                                                                            0.032                                                                            0.0079                                                                            17.0                                                                             0.118                                                                            0.001                                                                            0.0020                                                                            0.0053                                                                            -- 0.0676                                                                             118                         37 0.012                                                                            0.32                                                                             0.25                                                                             0.022                                                                            0.007                                                                            0.025                                                                            0.0056                                                                            16.9                                                                             0.283                                                                            0.001                                                                            0.0001                                                                            0.0022                                                                            1.01                                                                             0.0704                                                                             283                         38 0.010                                                                            0.19                                                                             0.30                                                                             0.021                                                                            0.003                                                                            0.036                                                                            0.0091                                                                            17.3                                                                             0.309                                                                            0.001                                                                            0.0009                                                                            0.0018                                                                            -- 0.0764                                                                             309                         39 0.005                                                                            0.15                                                                             0.22                                                                             0.019                                                                            0.010                                                                            0.010                                                                            0.0071                                                                            30.5                                                                             0.130                                                                            0.001                                                                            0.0009                                                                            0.0026                                                                            -- 0.0484                                                                             130                         40 0.006                                                                            0.18                                                                             0.22                                                                             0.026                                                                            0.002                                                                            0.025                                                                            0.0069                                                                            51.0                                                                             0.129                                                                            0.001                                                                            0.0008                                                                            0.0017                                                                            -- 0.0516                                                                             129                         41 0.009                                                                            0.29                                                                             0.26                                                                             0.021                                                                            0.005                                                                            0.020                                                                            0.0083                                                                             7.1                                                                             0.143                                                                            0.001                                                                            0.0010                                                                            0.0012                                                                            -- 0.0692                                                                             142                         42 0.006                                                                            0.28                                                                             0.33                                                                             0.022                                                                            0.005                                                                            0.060                                                                            0.0093                                                                            17.9                                                                             0.001                                                                            0.171                                                                            0.0009                                                                            --  -- 0.0612                                                                              <1                         43 0.011                                                                            0.30                                                                             0.16                                                                             0.015                                                                            0.006                                                                            0.033                                                                            0.0058                                                                            18.2                                                                             0.001                                                                            0.018                                                                            0.0011                                                                            --  -- 0.0672                                                                              <1                         __________________________________________________________________________

r-value, Δr

A test specimen of JIS No. 5 is cut out from the steel sheet in arolling direction, a direction of 45° with respect to the rollingdirection or a direction of 90° with respect to the rolling direction. Auniaxial tensile prestrain of 5-15% is applied to each of these testspecimens, during which a Lankford value in each direction is measuredfrom a ratio of lateral strain and thickness strain and calculatedaccording to the following equation:

    r=(r.sub.L +2r.sub.D +r.sub.T)/4

    Δr=(r.sub.L -2r.sub.D +r.sub.T)/2

wherein r_(L), r_(D) and r_(T) show Lankford values in the rollingdirection, direction of 45° with respect to the rolling direction anddirection of 90° with respect to the rolling direction, respectively.

Resistance to secondary work brittleness

A cup-shaped test specimen subjected to deep drawing at a drawing ratioof 2 is held at a particular temperature of -100° C.-20° C., andthereafter an impact load is applied to a head portion of the cupaccording to a drop weight test (weight: 5 kg, dropping difference: 0.8m), during which a crack creating temperature is measured from thepresence or absence of brittle crack at a sidewall portion of the cup.

In each of all steels, the test is conducted with respect to twospecimens for every temperature interval of 5° C. A temperature whenbrittle cracking is created in one of the two specimens is the crackcreating temperature.

Ductility of weld portion

The cold rolled steel sheet (thickness: 0.7 mm) is welded through TIGwelding method, from which is taken out a strip-shaped test specimen of15 mm×70 mm arranging a weld portion in center. The test specimen issubjected to a repetitive bending test (see FIG. 3) repeatingbending--returning operation 20 times, during which the occurrence ofcracking from the weld portion is observed. This test was carried outwith respect to 20 specimens of each of the test steels, and the crackcreating ratio was measured from the number of cracked specimens.

The test results are shown in Table 4.

                                      TABLE 4                                     __________________________________________________________________________       Diameter   Crack Bending     Diameter   Crack Bending                         of cold    creating                                                                            crack of    of cold    creating                                                                            crack of                     Steel                                                                            rolling                                                                            r-    tempera-                                                                            bead                                                                              Re-  Steel                                                                            rolling                                                                            r-    tempera-                                                                            bead                                                                              Re-                      No roll(mm)                                                                           value                                                                            Δr                                                                         ture (°C.)                                                                   (%) marks                                                                              No roll(mm)                                                                           value                                                                            Δr                                                                         ture (°C.)                                                                   (%) marks                    __________________________________________________________________________    1  180  1.72                                                                             0.14                                                                             -70   --  Accept-                                                                            23 180  1.82                                                                             0.03                                                                             -65   --  Accept-                  2  180  1.76                                                                             0.11                                                                             -75   --  able 24 180  1.79                                                                             0.05                                                                             -70   0   able                     3  180  1.65                                                                             0.12                                                                             -60   --  Example                                                                            25 180  1.86                                                                             0.03                                                                             -70   0   Example                  4  300  1.68                                                                             0.04                                                                             -65   --       26 180  1.61                                                                             0.10                                                                             -65   0                            5  180  1.63                                                                             0.09                                                                             -65   --       27 180  1.81                                                                             0.06                                                                             -70   0                            6  180  1.65                                                                             0.07                                                                             -75   --       28 180  1.90                                                                             0.09                                                                             -65   0                            7   80  1.63                                                                             0.07                                                                             -80   --       29 180  1.73                                                                             0.10                                                                             -60   5                            8  180  1.59                                                                             0.10                                                                             -80   --       30 180  1.69                                                                             0.15                                                                             -65   0                            9  180  1.58                                                                             0.13                                                                             -85   --       31 180  1.79                                                                             0.12                                                                             -70   0                            10  80  1.56                                                                             0.24                                                                             -55   --       32 180  1.61                                                                             0.41                                                                             -60   --  Compara-                 11  80  1.62                                                                             0.11                                                                             -60   --       33 180  1.60                                                                             0.42                                                                              -5   --  tive                     12 180  1.53                                                                             0.14                                                                             -50   --       34  80  1.47                                                                             0.45                                                                             -45   --  Example                  13  80  1.53                                                                             0.19                                                                             -55   --       35  80  1.45                                                                             0.41                                                                             -55   --                           14 180  1.55                                                                             0.14                                                                             -50   --       36  80  1.41                                                                             0.43                                                                             -60   --                           15 300  1.56                                                                             0.15                                                                             -55   --       37 180  1.63                                                                             0.41                                                                              15   --                           16 180  1.53                                                                             0.17                                                                             -50   --       38 300  1.45                                                                             0.50                                                                             -40   --                           17 180  1.73                                                                             0.11                                                                             -75   --       39 180  1.38                                                                             0.43                                                                             -35   --                           18 180  1.95                                                                             0.03                                                                             -75   --       40 180  1.25                                                                             0.63                                                                             -25   --                           19 180  2.01                                                                             0.02                                                                             -80   --       41 180  1.68                                                                             0.43                                                                             -55   --                           20 180  1.80                                                                             0.04                                                                             -70   --       42 180  1.28                                                                             0.50                                                                             -40   30                           21 180  1.85                                                                             0.02                                                                             -70   --       43 180  0.93                                                                             0.71                                                                             -40   30                           22 180  1.76                                                                             0.05                                                                             -65   --                                                        __________________________________________________________________________

As seen from Table 4, the steel sheets according to the inventionexhibit properties that the r-value is not less than 1.5, Δr is not morethan 0.3 and the crack creating temperature indicating the resistance tosecondary work brittleness is not higher than -50° C., so that they haveexcellent deep-drawing formability and resistance to secondary workbrittleness as compared to the comparative examples.

Furthermore, in the steel sheets containing Se according to theinvention, the cracking ratio of bead is not more than 10% in additionto the above properties.

EXAMPLE 2

Among the steels shown in Table 1, each of steel Nos. 1 and 6 was meltedin a convertor and subjected to secondary refining to obtain a steelslab, which was then heated to 1250° C. and hot rolled to obtain a hotrolled sheet having a thickness of 4.0 mm. The hot rolled sheet wasrendered into a cold rolled sheet having a thickness of 0.7 mm throughannealing of hot rolled sheet (800°-950° C.)--pickling--coldrolling--annealing of cold rolled sheet (800°-950° C.)--pickling. Inthis case, the cold rolling step of from 4.0 mm→0.7 mm in thickness(total reduction: 82.5%) was divided into a cold rolling stage I(thickness: 4 mm→X mm) and a cold rolling stage II (thickness: X mm→0.7mm), and the rollings of these stages were carried out under variousroll diameter and reduction conditions. A test specimen was taken outfrom the resulting steel sheet and then subjected to the same tests asin Example 1 for the evaluation of the properties. The results are shownin Table 5 together with the rolling conditions.

                                      TABLE 5                                     __________________________________________________________________________                         Steel No: 1                                                                              Steel No: 6                                   Cold rolling condition     Crack      Crack                                   Stage I     Stage II       creating   creating                                   Roll Reduc-                                                                            Roll Reduc-    tempera-   tempera-                                Run                                                                              diameter                                                                           tion                                                                              diameter                                                                           tion                                                                              r-    ture r-    ture                                    No (mm) (%) (mm) (%) value                                                                            Δr                                                                         (°C.)                                                                       value                                                                            Δr                                                                         (°C.)                            __________________________________________________________________________    1   80  82.5                                                                              --   --  1.70                                                                             0.24                                                                             -70  1.62                                                                             0.12                                                                             -75                                     2  180  20.0                                                                              80   78.2                                                                              1.70                                                                             0.23                                                                             -70  1.63                                                                             0.11                                                                             -75                                     3  180  35.0                                                                              80   73.1                                                                              1.81                                                                             0.12                                                                             -70  1.70                                                                             0.07                                                                             -75                                     4  180  50.0                                                                              80   65.0                                                                              1.82                                                                             0.10                                                                             -70  1.70                                                                             0.06                                                                             -75                                     5  180  82.5                                                                              --   --  1.85                                                                             0.08                                                                             -75  1.71                                                                             0.05                                                                             -75                                     6  300  35.0                                                                              80   73.1                                                                              1.75                                                                             0.13                                                                             -75  1.70                                                                             0.06                                                                             -80                                     __________________________________________________________________________

As seen from Table 5, all of the steel sheets have more excellentdeep-drawing formability and resistance to secondary work brittleness.

INDUSTRIAL APPLICABILITY

As mentioned above, the chromium steel sheets according to the inventionhave press formability, which has not been obtained in the conventionalchromium steel sheet, i.e. excellent deep-drawing formability andresistance to secondary work brittleness, which are useful in the pressforming. In the chromium steel sheets according to the invention,therefore, it is possible to conduct severer deep drawing forkitchenwares such as deep drop sinks and the like, automobile parts suchas fuel cases and the like, and also it is possible to prevent theoccurrence of brittle cracking in subsequent secondary working.

We claim:
 1. An iron-chromium steel sheet having excellent pressformability consisting essentially of:C: not more than 0.03 wt %, Si:not more than 1.0 wt %, Mn: not more than 1.0 wt %, P: not more than0.05 wt %, S: not more than 0.015 wt %, Al: not more than 0.10 wt %, N:not more than 0.02 wt %, Cr: not less than 11 wt % but not more than 60wt %, Ti: 4(C+N)-0.5 wt %, Nb: 0.003-0.020 wt %, B: 0.0002-0.005 wt%,and the balance being Fe and inevitable impurities.
 2. Aniron-chromium steel sheet having excellent press formability consistingessentially of:C: not more than 0.03 wt %, Si: not more than 1.0 wt %,Mn: not more than 1.0 w %, P: not more than 0.05 wt %, S: no more than0.015 wt %, Al: not more than 0.10 wt %, N: not more than 0.02 wt %, Cr:not less than 11 wt % but not more than 60 wt %, Ti: 4(C+N)-0.5 wt %,Nb: 0.003-0.020 wt %, B: 0.0002-0.005 wt %, Mo: 0.01-5.0 wt %,and thebalance being Fe and inevitable impurities.
 3. An iron-chromium steelsheet having excellent press formability consisting essentially of:C:not more than 0.03 wt %, Si: not more than 1.0 wt %, Mn: not more than1.0 w %, P: not more than 0.05 wt %, S: no more than 0.015 wt %, Al: notmore than 0.10 wt %, N: not more than 0.02 wt %, Cr: not less than 11 wt% but not more than 60 wt %, Ti: 4(C+N)-0.5 wt %, Nb: 0.003-0.020 wt %,B: 0.0002-0.005 wt %, Ca: 0.0005-0.01 wt %,and the balance being Fe andinevitable impurities.
 4. An iron-chromium steel sheet having excellentpress formability consisting essentially of:C: not more than 0.03 wt %,Si: not more than 1.0 wt %, Mn: not more than 1.0 w %, P: not more than0.05 wt %, S: no more than 0.015 wt %, Al: not more than 0.10 wt %, N:not more than 0.02 wt %, Cr: not less than 11 wt % but not more than 60wt %, Ti: 4(C+N)-0.5 wt %, Nb: 0.003-0.020 wt %, B: 0.0002-0.005 wt %,Se: 0.0005-0.025 wt %,and the balance being Fe and inevitableimpurities.
 5. An iron-chromium steel sheet having excellent pressformability consisting essentially of:C: not more than 0.03 wt %, Si:not more than 1.0 wt %, Mn: not more than 1.0 w %, P: not more than 0.05wt %, S: no more than 0.015 wt %, Al: not more than 0.10 wt %, N: notmore than 0.02 wt %, Cr: not less than 11 wt % but not more than 60 wt%, Ti: 4(C+N)-0.5 wt %, Nb: 0.003-0.020 wt %, B: 0.0002-0.005 wt %, Mo:0.01-5.0 wt %, Ca: 0.0005-0.01 wt %,and the balance being Fe andinevitable impurities.
 6. An iron-chromium steel sheet having excellentpress formability consisting essentially of:C: not more than 0.03 wt %,Si: not more than 1.0 wt %, Mn: not more than 1.0 w %, P: not more than0.05 wt %, S: no more than 0.015 wt %, Al: not more than 0.10 wt %, N:not more than 0.02 wt %, Cr: not less than 11 wt % but not more than 60wt %, Ti: 4(C+N)-0.5 wt %, Nb: 0.003-0.020 wt %, B: 0.0002-0.005 wt %,Mo: 0.01-5.0 wt %, Se: 0.0005-0.025 wt %,and the balance being Fe andinevitable impurities.
 7. An iron-chromium steel sheet having excellentpress formability consisting essentially of:C: not more than 0.03 wt %,Si: not more than 1.0 wt %, Mn: not more than 1.0 w %, P: not more than0.05 wt %, S: no more than 0.015 wt %, Al: not more than 0.10 wt %, N:not more than 0.02 wt %, Cr: not less than 11 wt % but not more than 60wt %, Ti: 4(C+N)-0.5 wt %, Nb: 0.003-0.020 wt %, B: 0.0002-0.005 wt %,Ca: 0.0005-0.01 wt %, Se: 0.0005-0.025 wt %,and the balance being Fe andinevitable impurities.
 8. An iron-chromium steel sheet having excellentpress formability consisting essentially of:C: not more than 0.03 wt %,Si: not more than 1.0 wt %, Mn: not more than 1.0 w %, P: not more than0.05 wt %, S: no more than 0.015 wt %, Al: not more than 0.10 wt %, N:not more than 0.02 wt %, Cr: not less than 11 wt % but not more than 60wt %, B: 0.0002-0.005 wt %, Mo: 0.01-5.0 wt %, Ca: 0.0005-0.01 wt %, Se:0.0005-0.025 wt %, Ti: 4(C+N)-0.5 wt %, Nb: 0.003-0.020 wt %,and thebalance being Fe and inevitable impurities.
 9. An iron-chromium steelsheet according to anyone of claims 2, 5, 6 and 8, wherein Mo content is0.1-3.0 wt %.
 10. An iron-chromium steel sheet according to anyone ofclaims 1 to 9, wherein a relationship between Ti content and Nb contentsatisfies Ti/Nb≧7.